The goal of this research is to test models for mechanisms of direct radiation damage to DNA for radiations of varying LET and damage repair by radioprotective agents. Recent models for DNA damage and thiol autoxidation have been developed in our laboratory. The first describes DNA radiation events from the initial DNA ion radicals to the formation of secondary radicals and finally to diamagnetic products. The second describes the autoxidation cycle of thiols from thiyl radical to sulfonyl peroxyl radical. Our investigations will test the effect of thiols on DNA free radical processes in both oxic and anoxic systems in order to seek a better understanding of the chemical oxygen enhancement effect and thiol radioprotection. These studies, which are performed under conditions that emphasize the direct effect of radiation, will employ electron spin resonance spectroscopy (ESR), theoretical modeling using ab initio molecular orbital theory (MO calculations), and gas chromatography--mass spectroscopy (GC-MS) techniques. The effects of high LET heavy ion radiation on DNA and its repair by thiols will be investigated by use of the National Superconducting Cyclotron at Michigan State University. Specifically, in Aim I investigations of radical mechanisms important to DNA radioprotection and the chemical oxygen enhancement effect will be investigated. Included are: 1. The reaction of sugar radicals with thiols in anoxic and oxic systems. 2. The investigation of the attack of sulfur- oxygen (sulfoxyl) radicals on DNA and DNA components. 3. The investigation of the reaction of specific DNA base radicals with thiols to experimentally ascertain which specific radicals react with thiols. In Aim II ab initio MO calculations will also be employed to predict which of the many known DNA radical intermediates are able to participate in radioprotection by thiols for comparison to and elucidation of experimental results. Use of isodesmic reactions will allow for calculation of accurate reactions energies. In Aim III investigations of high LET irradiation on DNA-thiol systems will test the hypothesis that damage by high LET radiation is more complex than that caused by low LET radiation and that it is this complex damage which results in the decrease in OER with LET. In Aim IV samples irradiated and investigated by ESR in Aims I and III will be analyzed for DNA base damage and base release by GC-MS and HPLC. Product analyses will be performed jointly with Wake Forest University. Results of these analyses will serve to test and further elucidate mechanisms proposed by ESR experiments.